1. Field of the Invention
This invention relates to an apparatus for sorting an opaque foreign article from among transparent bodies, and more particularly to an apparatus for sorting an opaque foreign article such as a pebble or a ceramic piece from among a large number of transparent bodies such as glass pieces of recovered cullets.
2. Description of the Prior Art
An apparatus of the type mentioned has been proposed by the inventor of the present invention and is disclosed in European Patent Application Publication No. 0 413 522 A2. In the apparatus, objects including transparent bodies in which opaque foreign articles may mix are transported on a conveyor and then allowed to drop individually from the conveyor, and then while they are dropping, they are scanned horizontally along a straight line with a linearly polarized laser beam and reflected light is detected by means of a CCD (charge coupled device) image sensor by way of a polarizing filter. Whether the object scanned is a transparent body or an opaque foreign article is judged from outputs of CCDs of the image sensor.
In particular, when the object scanned is a transparent body, most of the laser beam passes through the object while only a small part of the laser beam is reflected by the object, and since such reflected light remains as linearly polarized light, it is intercepted by the polarizing filter. On the other hand, when the object scanned is an opaque foreign article, the laser beam is irregularly reflected by the object to change into circularly polarized light and most of it passes through the polarizing filter so that it falls incident on the image sensor. Accordingly, when the object scanned is a transparent body, a CCD of the image sensor will provide a comparatively low output level, but when the object scanned is an opaque foreign article, such CCD will provide a comparatively high output level. Therefore, a transparent body and an opaque foreign article can be distinguished from each other in accordance with a difference between output levels of a CCD. Then, in case the object has been judged as an opaque foreign article, air will be jetted to the object during dropping from the conveyor to blow off and separate it from the other transparent bodies.